1. Field of the Invention
The present invention relates to a focus detection system for performing focus adjustment.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2000-338392 discloses a focus detector that re-forms a first-order image of an object, the first-order image being formed by an imaging lens, onto a light sensor, and that detects a focus using a pair of sensor outputs formed by a pencil of rays passing through different regions of an exit pupil of the imaging lens.
In this focus detector, an image of an object formed by the imaging lens is re-formed onto the light sensor with an absolute-value magnification of less than one. Therefore, a focus is advantageously detected in a large region of an image of an object with a small light sensor. If a micro lens is installed in the light sensor, the amount of light received is increased and the signal-to-noise ratio (SNR) is improved, and therefore, focus detection with good accuracy is expected.
Japanese Patent Laid-Open No. 55-111927 discloses a focus detector that receives a first-order image of an object formed by an imaging lens via a micro lens and that detects a focus using a pair of sensor outputs formed by a pencil of rays passing through different regions of an exit pupil of the imaging lens.
In this focus detector, an optical system for image reformation is not necessary and the light sensor is simply arranged adjacent to a first imaging face of the imaging lens. Therefore, one advantage obtained is that the structure becomes much simpler.
Focus detectors of the type disclosed in Japanese Patent Laid-Open No. 2000-338392 mentioned above commonly use an aperture stop when an image of an object formed by the imaging lens is re-formed onto the light sensor. Inversely projecting the aperture stop by a field lens in an exit pupil of the imaging lens restricts a region where a pencil of rays to be used in focus detection passes through the exit pupil. As a result, for a second-order image of the object formed through the same aperture stop, a region where a pencil of rays passes through the exit pupil of the imaging lens is constant at any position.
In imaging lenses for use in photography, two exit windows determined by two apertures (e.g., two lens surfaces) in many constituent lens elements determine a pencil of light in the outer region. A pencil of rays forming an image on the optical axis is widest, and another pencil of rays becomes narrower as it approaches the edges of an image circle due to vignetting.
In order to detect a focus with high accuracy even when the brightness of an object to be subjected to focus detection is low, it is effective to use as much of a pencil of rays forming an image of the object as possible for focus detection and to increase the amount of light introduced to a light sensor used for focus detection.
From this viewpoint, when a pencil of rays forming an image of an object is compared with a pencil of rays used in focus detection, only a small amount of the pencil of rays forming an image is used in focus detection around the optical axis, whereas much of the pencil of rays forming an image is used in focus detection at the edges of the image circle. In other words, a pencil of rays is not effectively used around the optical axis.
The focus detector disclosed in Japanese Patent Laid-Open No. 55-111927 requires a light sensor larger than at least a region used for focus detection. Therefore, when the focus detector is used in a film camera employing 35 mm film (135 film) or 120 film, a digital camera employing a large image sensor, or the like, a required focus-detection region is large because an image-capture screen is large. As a result, the light sensor used for focus detection is inevitably large and the focus detector is thus significantly expensive. On the other hand, if a small light sensor is used, a camera will have only a small focus-detection region and have poor usability, and this camera will be impractical.